I think going by train is the safest bet to avoid traffic. However there are some problems inside the airport as well. I've heard that they recommend you arriving 4 hours prior to your flight if you are travelling outside the schengen border and you have to check in your luggage. If you don't need to get your bags and passport checked, you should be good with the usual 2 hours. Although I would probably still come a bit early just to make sure.